Alyson Hannigan

It’s Wednesday at the Century City, Calif., sets of CBS’ Monday sitcom "How I Met Your Mother," and though cast and crew are battling a collective case of the sniffles, spirits are high because it’s cupcake day, with unfrosted cupcakes and a selection of frostings there for the taking.

Jason Segel — who plays one half of the show’s engaged couple, Marshall and Lily (Alyson Hannigan) — swings by, raves about the cupcakes, frosts one, then feigns shaking hands from a distance.

"Don’t get close," he warns.

On tap this day is the filming of a scene for an episode called "Marshall-opoly," in which the convoluted rules of a made-up game lead to the main characters revealing their most embarrassing moments.

One of the stories recalls the show’s pilot, in which single architect Ted (Josh Radnor) met and instantly fell for single reporter Robin (Cobie Smulders), an experience he shared with law student Marshall and teacher Lily, and with his womanizing, suit-wearing best pal, Barney (Neil Patrick Harris).

But Robin is not the "Mother" in question, as viewers learned from the show’s framing device, in which unseen Future Ted (voiced by Bob Saget), in the year 2030, tells his teenage children what producers hope will be a many-seasons-long tale about how he met their mother — and their "Aunt Robin."

A growing critical and ratings hit, the show examines love from three angles: committed, wanting to be committed, and utterly uncommitted.

As show lore goes, Marshall and Lily are based on co-creator Craig Thomas and his wife, Rebecca, and Ted is based on single co-creator Carter Bays.

"I met the love of my life in college," Thomas says, "which is unusual, and this show is unusual because it features two guys who want to be in committed relationships, one who is, and the other who’s looking."

"Marshall’s like me," Segel reveals. "He wants to love only one woman."

"It’s a great gender reversal," Radnor says, "because the guys are sensitive and ready to commit while the girls are tough, especially Robin. She’s ostensibly not looking for a relationship, but she might be turning the corner.

"Lily’s very efficient and perhaps a little more mature than her betrothed. In many ways, it’s so traditional, but it’s also flipped on its head. It takes these elements that feel familiar then pulls the rug out from under them."

Hannigan came into the show because Thomas’ wife would only allow him to base a character on her if the former "Buffy the Vampire Slayer" regular was cast in the part.

"Marshall and Lily are based on Rebecca and Craig," Hannigan says. "A lot of the stories they do are based on their life in New York. It is very personal and real. I don’t think you would care as much otherwise."

Wed to her former "Buffy" co-star Alexis Denisof (who, according to his wife, will guest-star in a couple of episodes of "How I Met Your Mother"), Hannigan also believes the whole show rings true to life.

"I definitely think it feels real," she says. "And it’s really nice to see that guys go through some confusion as well. It’s nice to see that part of the story. It does exist. I have friends that are searching. So it’s refreshing."

Smulders has her foot up between takes, having her co-stars sign her cast. She injured an ankle while shooting a laser-tag scene — but was walking fine at a party a month later, so all is well. Since she has such good chemistry with Radnor, it’s sad to realize that, no matter what, Robin’s not the one for Ted.

"As time goes by," Smulders says, "she realizes what a great catch he is. But it’s one of those too-late moments, unfortunately. It was there and then it was gone, like that."

Bays, who’s still searching for his one and only, waxes philosophical during another break. "It’s almost an existential concept: How do you go about searching for what’s essentially divine intervention? How do you go out and seek the thunderbolt to hit you? If you decide, I’m ready to settle down, fall in love, get married, how do you make that happen? Being unmarried, that’s a question that’s constantly perplexing.

"Ted is an architect, so much of his life is about designing something, building something. If you plan it out well, it’ll come to pass, and this is one thing in his life that he can’t design."

Standing just to one side of this search for love is Barney, who proclaims he’s perfectly satisfied to nail as many chicks as possible without long-term repercussions. But aside from his roving eye and fondness for snazzy suits, not much is known about the man.

"There are a lot of mysteries about Barney," Bays says. "We don’t know what he does for a living. We’ve never seen his apartment or his family. Hopefully we’ll have many years to answer those questions. He’s the monster in the woods."

Hannigan reveals "How I Met Your Mother" has passed muster with one particularly harsh critic, "Buffy" creator Joss Whedon, who started his TV career writing such sitcoms as "Roseanne" and "Parenthood."

"That’s why he’s so jaded," Hannigan says, "and why he’s so hard to please. I never thought I’d be on a show that Joss liked, but I’ve done that. I can’t believe it. He said, ’I’m loving it.’ He said it’s on their TiVo list, for season pass."

Whedon recently made a cameo appearance on UPN’s "Veronica Mars," on which Hannigan has a recurring role. Might he do the same for "How I Met Your Mother"?

"We’ve got to work on his cameo," Hannigan says. "That would be awesome. The fact that he likes this show is ridiculously huge."